1. Field of Invention
The present invention relates to photothermographic materials and image-forming methods using photothermographic materials and, particularly, to a photothermographic material containing a silver halide emulsion having a high silver iodide content and an image-forming method using the photothermographic material. More specifically, the present invention relates to a photothermographic material in which sensitivity is substantially improved, fog is low, and storability of images after development is excellent, and an image-forming method using the photothermographic material.
2. Description of the Related Art
In recent years, in the fields of medical diagnosis and printing plate forming from the standpoints of environmental protection and space saving, photographic development processing performed in a dry state has been strongly demanded. These fields have seen an increase in digitalization, as well as systems in which image information is fed into a computer and stored and then optionally processed, transmitted, and outputted, by a laser image setter or a laser imager, to a photosensitive material at a position required, and duly developed to form images. Photosensitive materials are now expected to have a capability of being recorded by laser exposure of high illuminance, and a capability to form clear black images having high resolution and sharpness. As for digital imaging recording materials, various types of hard copy systems using pigments and dyes, such as ink jet printers and electronic photographic systems, are available as ordinary image-forming systems. However, none of these hard copy systems is satisfactory either in terms of aspects of image quality which determine the diagnostic capacity of the images as images for medical uses, such as for example sharpness, graininess, gradation, and color tone, or in respect of the recording speed (i.e. sensitivity of the images), nor have any of these systems yet attained a level of which it can be used to replace the conventional wet-development-type silver salt film used in medical diagnosis.
On the other hand, thermally developable image forming systems utilizing organic silver salts are already known in, for example, U.S. Pat. Nos. 3,152,904, and 3,457,075, and in D. H. Klosterboer, “Thermally Processed Silver Systems” (see Imaging Processes and Materials, Neblette, 8th Ed. compiled by J. Sturge, V. Walworth and A. Shepp, Chap. 9, page 279, 1989). The photothermographic material comprises an image-forming layer in which a photosensitive silver halide, a reduction agent, a reducible silver salt (for example, organic silver salt) and, optionally, a color toning agent for controlling the color tone of silver are ordinarily dispersed in a binder matrix.
When the photothermographic material is exposed and heated at a high temperature (for example, 80° C. or more), a monochromatic black silver image is produced by an oxidation-reduction reaction between the silver halide, or the reducible silver salt (functioning as an oxidizing agent), and the reduction agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image of the silver halide generated by such exposure. Therefore, monochromatic silver images are formed in areas of the material which have been exposed. Such photothermographic materials are disclosed in many documents (see, for example, U.S. Pat. No. 2,910,377 and JP-B No. 43-4924) and, further, the Fuji Medical Dry Imager FM-DP L has been commercially available for practical use as an image-forming system for medical diagnosis. However, since such image-forming systems utilizing organic silver salt have no fixing step that allows the silver halide to remain in a layer after thermal development, the systems have serious drawbacks.
One drawback has been image storability after development processing, particularly, printout degeneration caused by exposure to light. A method utilizing silver iodide is known as a means to alleviating printout degeneration. Compared with silver bromide or silver iodobromide having an iodine content of 5 mol % or less, silver iodide has a property which makes it extremely difficult for print out to be generated and in consequence silver iodide has the potential to drastically solve the problem. However, a silver iodide grain which has been known so far has a sensitivity so low that it is still a long way from being capable of being applied to a practical system. Further, an additional problem inherent in the grain has been that, when, in order to enhance sensitivity, a step has been taken to prevent a reunion of a photon and a hole, a property excellent in preventing printout has thereby been lost.
As a method of increasing sensitivity of a silver iodide photographic emulsion, it has been recorded in literature that sensitization can be achieved either by using halogen receptors such as sodium nitrite, pyrogallol, and hydroquinone, by immersion in a silver nitrate aqueous solution, or by sulfur sensitization at pAg 7.5 (see, for example, The Journal of Photographic Science, Vol. 8, p. 119, (1960), The Journal of Photographic Science, Vol. 28, p. 163, (1980), and Photographic Science and Engineering, Vol. 5, page 216 (1961). However, the sensitizing effects at these halogen receptors on photothermographic materials intended by the present invention has been minimal and conspicuously inadequate.
Another drawback is that, because of light scattering caused by residual silver halide, a layer becomes clouded and falls into a state between translucence and opacity, thus causing a deterioration in images. In order to solve this problem, a practical measure has been adopted in which photosensitive silver halide is mae into a state of fine grain (from 0.15 μm to 0.08 μm in an area actually used) and then the quantity of photosensitive silver halide added is reduced as far as possible, thus minimising cloudiness. However, a compromise measure of this kind results in a further decrease in sensitivity and has not completely solved the problem of cloudiness. Opacity has still remained in the layer and haze has remained imparted therein.
In the case of a wet developent processing system, residual silver halide is removed by performing, after the development process, a process using a fixing liquid containing a silver halide medium. As regards such silver halide media, various types of inorganic and organic compounds which can form a complex with a silver ion are known.
In the case of a dry thermal development process also, incorporation of a similar fixing measure has in the past been attempted. For example, a method has been proposed in which a compound capable of forming a complex with a silver ion is contained in a layer to allow the silver halide to be solubilized by thermal development (ordinarily known as fixing: see JP-A No. 8-76317). However, this method is a system associated with silver bromide or silver chlorobromide, and post-heating is also necessary for the fixing, requiring, as a heating condition, a temperature as high as from 155° C. to 160° C., in which it is difficult to conduct the fixing. Further, another method has been proposed in which a separate sheet (fixing sheet) containing a compound capable of forming a complex with a silver ion is prepared and, then, after the photothermographic material is thermally developed to form an image, the photothermographic material developed is superposed on the fixing sheet and heated to dissolve the remaining silver halide. Thereafter, the silver halide which has been dissolved is removed (see JP-A No. 9-166845). However, since in this method two sheets are used, the treatment process becomes complicated and it has proved difficult to secure operational stability. Further, it is necessary to dispose of the fixing sheet after use in the fixing process. Waste is thereby generated and from a point of view of practicability this becomes a barrier.
Further, as for fixing methods in thermal development other than those described above, a method has been proposed in which a fixing agent of a silver halide is first microencapsulated and then the fixing agent is released to act at the time of thermal development (JP-A No. 8-82886). However, for this method, it is difficult to arrive at a design that allows the fixing agent to be effectively released. Still another method has been proposed in which fixing is performed by using a fixing liquid after thermal development (JP-A Nos. 51-104826, and 62-133454). However, in this method a wet-type treatment needs to be performed, and thus this method is not appropriate for a completely dry process.
As described above, there have been known drawbacks to various known methods of improving opacity of the layer that there have invariably been obstacles in the way of putting them to practical use.
Further, although it is a liquid development process system, a method is known in which a silver salt be deposited in an epitaxial growth manner on a silver halide grain which acts a host. Another method is also known in which higher sensitivity is obtained by making a silver halide have a dislocation line (see, for example, JP-A Nos. 59-119344 (corresponding to U.S. Pat. No. 5,017,469) and 59-119350 (corresponding to U.S. Pat. No. 4,435,499), and Japanese Patent No. 2664272).
However, in a silver halide photosensitive material which it is intended to subject to a liquid development process, a silver image is generally formed by reducing silver halide by means of a developing agent (reduction agent) contained in a liquid of the liquid development process. Alternatively, a color image is formed by making use of an oxidized form of the developing agent generated as a by-product. In either case a fundamental reaction is a reduction of the silver halide by the developing agent. On the other hand, in the photothermographic material, the silver halide forms a latent image only by being exposed to light and, while a silver ion supplied from a non-photosensitive organic silver salt is reduced, the silver halide itself is not reduced by the reduction agent. In the case of the liquid development process reduction agents are ionic reduction agents such as hydroquinone, and p-phenylene diamines, while in the case of a photothermographic material reduction agents are hindered phenol derivatives, generally known as radical reaction agents.
As described above, in the photosensitive material for the liquid development process and the photothermographic material, the mechanisms of development reactions (reduction reaction) are completely different from each other, and the systems of compounds used in the respective materials are also completely different. Accordingly, it can not be assumed that a compound which has been effective in a liquid development process will automatically also be effective in a photothermographic material. Further, when the compound is applied to the photothermographic material, it can not be predicted whether the same effects will be obtained or whether completely different effects can be expected moreover, it is not possible to recall an instance of a compound being applied to a photothermographic material using a high silver iodide emulsion and, it is therefore impossible to foresee the effects of such a compound on a photothermographic material.
On the other hand, an attempt has been proposed to apply the photothermographic material to a photosensitive material for photographing. The term “photosensitive material for photographing” as used herein does not mean a material in which image information is written in by a scanning exposure by means of, for example, laser light, but rather means a material in which an image is recorded by a planar exposure. Since conventionally photothermographic materials have been commonly used in the field of wet development photosensitive materials, a direct or an indirect X-ray film, a mammography film or the like in medical diagnosis applications, various types of plate-making films in printing applications, a recording film in an industrial application, a film for photographing by general-use cameras and the like are all known. For example, disclosed in patent documents have been a photothermographic material for an X-ray application making use of a blue fluorescent sensitizing paper of a both-sides-coated type (see, for example, Japanese Patent No. 3,229,344), a photothermographic material using a tabular grain of silver iodobromide (see, for example, JP-A No. 59-142539), and a photosensitive material for a medical diagnosis application which has a (100) main face and in which a tabular grain having a high silver chloride content is applied on each surface of a support (see, for example, JP-A No. 10-282602). Further, the photothermographic material of a both-sides-coated type has also been disclosed in patent documents other than those described above (see, for example, JP-A Nos. 2000-227642, 2001-22027, 2001-109101, and 2002-90941.). However, in these known examples, when a fine grain silver halide having a size of 0.1 μm or less is used, haze does not deteriorate, but sensitivity remains low and in consequence the photothermographic material is not able to stand up to practical use. On the other hand, when a silver halide grain having a size of 0.5 μm or more is used, image quality is severely damaged by a deterioration in haze and printout degradation both caused by residual silver halide, thus preventing the photothermographic material from standing up to practical use.
Although a photosensitive material using a tabular grain of silver iodide as a silver halide grain has been known in the wet development field (see, for example, JP-A Nos. 59-119344, and 59-119350), no example is known of a photothermographic material in such applications. The reasons for this are that are taken the photothermographic material has low sensitivity, that no effective sensitizing measures and that, in the case of thermal development, the technical barrier is raised even higher.
In order to use the photothermographic material in the application of the photosensitive material for photographing, a higher sensitivity is required and a level of image quality one-step higher than normal is required, with regard both to haze and to the aspects.